1. Technical Field
The present invention relates in general to integrated circuits and in particular to integrated memory circuits. Still more particularly, the present invention relates to integrated read only memory circuits.
2. Description of the Related Art
A common read only memory ("ROM") configuration utilized is an X-cell ROM having alternating column lines and bit lines, as illustrated in U.S. Pat. No. 4,758,989. Each column line is connected to adjacent bit lines by single-transistor memory cells that may be programmed very high threshold and thus permanently "off" or normal low threshold and conductive when the gate is high. In operation, signals from the column decoder circuit control select a column line that is pulled down to ground. Word lines turn on transistors connected to bit lines adjacent to the selected column line, creating a current path from a bit line to ground for those transistors that are programmed in an "on" state. Those bit lines having a path to ground drop in voltage from an initial pre-charged state, while those bit lines connected to transistors that are programmed very high threshold remain at the pre-charge voltage.
Voltage levels on the two bit lines adjacent to a selected column indicate the data in each corresponding memory cell. An X-cell ROM is inherently a by-2 device, since at least two bits are available, even if only one column line is selected.
ROMs may be programmed through threshold voltage programming, active area programming, or contact programming. Programming value of bits in ROMs may be accomplished by a latent heavy boron implant that raises the threshold of N-channel transistors. This technique, however, may be problematic if the breakdown voltage of the transistor is too low. Active area programming is the easiest from a design stand point. On the other hand, from a manufacturing point of view, active area programing is not so desirable. Programming is performed during one of the first steps in the processing steps required to construct the ROM. Often changes in programming may be required, and in such a case, this type of processing is a drawback especially if programming changes are required. Firmware changes required due to programming changes may result in the scrapping or abandonment of ROMs already past the early steps of processing.
Another option for setting bit values involves contact programing, which allows for the programming of the ROM at a later stage in manufacturing. This method, however, has a disadvantage of requiring more "real estate" or surface area than active area programming since each transistor representing a bit requires a separate contact. The amount of surface area utilized is critical in high density ROMs, such as ROM employed in microcomputers. Therefore, it would be desirable to have a ROM that may be programmed late in the manufacturing stage without requiring large amounts of surface area.